Evaluation of physico-chemical and mechanical properties of MTA-based root canal sealer

Aim: The aim of this study was to evaluate and compare the setting times (ST), flow (FW), radiopacity (RP), dimensional stability (DS), solubility (SB), and polymerization stress (PS) of the MTA Fillapex and AH Plus root canal sealers. Methods: The above qualities were tested according to ISO 6876/2001 standardization. The water used in the dimensional stability test was evaluated to verify the presence of released materials. For the photoelastic analysis, 20 photoelastic resin rings were prepared, and the root canal sealers were inserted. After 24 hours, the specimens were analyzed in a Polariscope. Data of the setting times, flow tests, radiopacity, dimensional stability, and solubility tests were submitted to a Kolgomorov–Smirnov test and then to a Student’s t-test at the 5% significance level. Results: The data derived from photoelastic analyses were submitted to an ANOVA and Tukey’s test with a significance level of 5%. MTA Fillapex and AH Plus complied with ISO 6876/2001. However, there were significant differences (p < 0.05) between the two cements for ST, FW, RP, DS, and SB. MTA Fillapex showed higher FW, SB, and PS when compared with AH Plus. Conclusions: MTA Fillapex and AH Plus complied with ISO 6876/2001 in terms of ST, FW, RP, DS, and SB. MTA Fillapex showed higher PS when compared to AH Plus.


Introduction
In general, the process of filling root canals requires an endodontic sealer and a core material. Currently, the most commonly used core material for filling root canals is the gutta-percha cone, which is universally accepted and regarded as the gold standard in obturation of the root canal system 1 . Nevertheless, the gutta-percha cone cannot adhere to the dentinal surface, so an endodontic sealer should provide good adhesion and flowability that can penetrate into the irregularities and fill the space between the gutta-percha cone and the root canal walls 2 . Therefore, the root canal sealer has as much or more importance than the core material in providing successful clinical outcomes 3 .
Root canal sealers are responsible for different functions during the filling of root canals: they lubricate and guide the settlement of the main cone, serve as a bonding agent between the cones and the canal walls, and fill the anatomic spaces that gutta-percha cannot achieve. In addition, they might have antibacterial properties, which is desirable, especially from a clinical viewpoint, because it is not uncommon that microorganisms flourish in the root canal system after treatment 4 .
The research on adhesive technology has contributed to determining a way to minimize leakage and thereby increase the sealing ability between the filling materials and the root walls 5 . Several sealers have been developed and studied for endodontic treatment. Among them is AH Plus, a resin-based cement that has important properties as a long-term sealant and an appropriate dimensional stability and high radiopacity. Moreover, it has a higher adhesion bond strength when compared with other sealers, and it has good biological properties 6,7 .
In recent years, Mineral Trioxide Aggregate (MTA) has received much recognition among all endodontics materials 8 not only because of its versatility but also because it can be used to recover teeth that have been deemed as nonrestorable due to extensive caries, developmental anomalies, and iatrogenic damage 9 . It was first described as a material for sealing off all of the pathways of communication between the root canal system and the external surface of the tooth and later as a root-filling material 7,10,11 . MTA is a powder composed of tricalcium silicate, bismuth oxide, dicalcium silicate, tricalcium aluminum, aluminum-tetracalcium ferric, and dehydrated calcium sulfate, which hardens when hydrated and becomes a colloidal gel with a pH 12.5 12,13 . In addition, MTA is rich in calcium ions, which are converted into calcium hydroxide in aqueous solutions and improve biocompatibility and promote the action of cementoblasts 14 .
MTA is a bioactive material 15 that has the ability to stimulate the release of cytokines from bone cells, indicating that it actively promotes hard tissue formation 16 , in addition to its antimicrobial properties and reduced cytotoxic effect 17 . The excellent properties of a mineral trioxide aggregate, including its biocompatibility, bioactivity, and osteoconductivity, have fostered a race to develop new calcium silicate-based endodontic sealers that incorporate MTA 18 . However, both the difficulty of manipulation and the short working time limit its use as a root canal sealer 19 . MTA formulation has been modified by adding polymers and other sub-stances to improve properties such as flowability, setting time, and bond strength, without changing its biocompatibility 20 . MTA Fillapex (Angelus, Londrina, PR, Brazil)-which comprises MTA, silica nanoparticles, silicate resin, natural resin, bismuth oxide, and pigments-came to be applied as a new silicate-based sealer 21 . It has suitable flow, good sealing, and low solubility 22,23 and is indicated for use in cold and warm root-filling techniques 24 .
Thus, the purpose of this study was to evaluate and compare the setting times, flow, radiopacity, dimensional stability, solubility, and photoelastic analyses of the MTA Fillapex (Angelus, Londrina, PR, Brazil) and AH Plus (Dentsply-De-Trey, Konstanz, Germany) root canal sealers. The hypothesis was that both materials should exhibit similar behaviors for all properties.

Setting Time
Setting time was evaluated following the International Standards Organization (ISO) specifications 6876:2001 25 . Ten copper circular molds (n = 10) with an internal diameter of 10.0 mm and a thickness of 2.0 mm were used. The sealers were mixed according to the manufacturer's directions. These molds were maintained at 37 ± 2 °C and a relative humidity of 95 ± 2%. Immediately and at 150 seconds after the onset of mixture, a 1/3-lb Gilmore-type needle with a flat end with a diameter of 2.0 ± 0.1 mm was carefully lowered vertically and onto the horizontal surface of each specimen in 15-minute intervals. Next, the 1-lb Gilmore-type needle was used to measure the final setting time. The needle tip was cleaned, and the process was repeated at 60-second intervals until no indentations were visible. The setting time that each sealer needed to reach this state was noted.

Flow Test
According to ANSI/ADA specification no. 57 26 , 0.5 mL of each cement was placed in the center of a glass plate with a disposable 5-mL syringe. At 180 seconds after the start of mixing, another glass plate was placed centrally on top of the sealer, followed by a weight with a mass of 120 g. Ten minutes after the commencement of mixing, the load was removed, and the major and minor diameters of the compressed discs were measured with a digital caliper (Mitutoyo MTI Corp., Tokyo, Japan). If the major and minor diameter discs were not uniformly circular or did not match within 1 mm, the test was repeated. The test was repeated until 10 cement discs of each group were obtained.

Solubility
A Teflon ring matrix of 1.0 mm in height and 15.0 mm in diameter was made, and 20 impressions were taken with polyvinylsiloxane. These molds were filled with fresh experimental sealers (n = 10) and covered with cellophane sheet. Next, the assembly was transferred to an incubator set at 95% relative humidity and 37 °C, left to stand for a period corresponding to three times the setting time, and was then removed.
The specimens were removed from the molds and measured, two by two, on a precision balance (JK-180, Chyo Balance Corp., Tokyo, Japan). The specimens were immersed in 7.5 mL of deionized water for 7 days at 37 °C in an incubator. Afterward, they were removed, rinsed, and stored in a desiccator for 24 hours and measured. The solubility of the sealer was determined by ascertaining the weight loss of each sample, using the mass difference between the original and the final mass 25 .

Dimensional Stability Test
A cylindrical Teflon matrix that was 6 mm in diameter and 12 mm tall was made, and 20 impressions were taken with polyvinylsiloxane. The molds were fixed with wax in a glass plate and filled with the experimental sealers (n = 10) and then covered with a polyester strip. The specimens were measured with a digital caliper (Mitutoyo, South America, São Paulo, Brazil) after they sat in an incubator at 37 °C.
The specimens were immersed in a flask with 2.24 mL of deionized water and stored in an incubator for 30 days. The specimens were then removed, dried, and measured again to obtain the final specimen length 25 . The water in which the samples were immersed was analyzed in a UV-visible spectrophotometer (BioMate TM 3, Thermo Fisher Scientific, Waltham, Massachusetts, United States) to evaluate the presence of material release.

Radiopacity
A Teflon mold that was 1 mm thick and 10 mm in diameter was made, and 20 impressions were taken with polyvinylsiloxane (Aquasil Ultra Smart Wetting, Dentsply Caulk, Milford, Delaware, United States). The sealers were poured into the impressions matrix on a glass plate covered with a cellophane sheet and another glass plate. The filled impressions were kept at 37 °C and 100% of the relative humidity until the cements completely set.
The matrix was removed after the sealer completely set, and the thicknesses of the specimens were confirmed with a digital caliper (Mitutoyo, South America, São Paulo, Brazil). Radiographs were taken with a radiographic unit (Electronic Spectro 70X, Dabi Atlante, Ribeirão Preto, Brazil) with 70 kV, 10 mA, an exposure time of 0.30 seconds, and a focus-film distance of 30 cm using a specific apparatus composed of two 30-cm plastic rulers and two metal clamps. Along with the specimens, an aluminum step wedge was positioned together to enable the analysis of the radiographic density. The scale comprised 19 aluminum increments with increasing thickness variations of 0.5 mm. Next, the radiographs were processed via a digital radiography system (Schick Technologies Inc., United States).
To determine the radiograph densities, one observer compared the sealer image with the scale. The evaluation was performed three times for each sample, and the average was taken to determine the level of radiopacity of each sample. The higher the level of the scale, the higher the radiopacity of each sealer 25 .

Photoelastic Analysis
Twenty photoelastic resin rings (PL2, Vishay measurements, Raleigh, United States) were made with the following measurements: a 5-mm internal diameter, a 15-mm external diameter, and a 2-mm depth. After the polymerization of the photoelastic resin, the inner walls were sandblasted with 50-mm aluminum oxide particles. The specimens were divided into two groups of 10 each, depending on the tested material. The root canal sealers were manipulated and poured into the photoelastic ring cavities. The specimens were stored at 37 °C for 24 hours. After this period, the specimens were removed from the incubator and analyzed in a Polariscope (PhotoStress, Vishay LF/Z-2, Malvern, United States). The data were obtained in MPa via Polariscope software (PSCalc, Vishay Measurements Group Inc., United States).

Statistical Analysis
Data of the setting time, flow test, radiopacity, dimensional stability, and solubility test were submitted to a nonparametric test, the Kolmogorov-Smirnov test, and then subjected to Student's t-test at the 5% significance level. The data for the photoelastic analysis were submitted to an ANOVA and Tukey's test with a significance level of 5%.

Setting Time
ISO 6876/2001 25 specifies that the curing time of a sealer must not be more than 10% of those stated by the manufacturers. The minimum setting times provided by the manufacturers of AH Plus and MTA Fillapex are 8 hours and 2 hours, respectively. Table 1 shows the initial and final setting times for each root canal sealer, as well as the comparisons by Student's t-test between the initial and final mean values of setting times. Both root canal sealers were consistent with ISO 6876/2001 standardization.

Flow Test
Root canal sealers were evaluated in accordance with ANSI/ADA specification no. 57 26 , which stipulates that cements should not display diameters less than 20 mm. The only differences between ADA 26 and ISO 6876/2001 25 standards are the volume analyzed and the minimum diameters of the spreads. Table 1 contains the mean diameter (in mm) of the circles obtained during the flow test. Both tested sealers  25 stipulates that the radiopacity should be higher than the radiopacity of 3 mm of aluminum. The tested root canal sealers had values above the ISO regulation. Table 1 shows the means of the radiopacity levels (mm of aluminum) for each root canal sealer. The statistical analysis demonstrated that AH Plus significantly had a higher radiopacity than MTA Fillapex.

Photoelastic Analysis
In the present study, photoelastic analysis was used to determine how the sealers could generate stress on root dentin. Table 1 shows the photoelastic means for the MTA Fillapex and AH Plus specimens. The statistical analysis demonstrated that MTA Fillapex produced a significantly higher strain than AH Plus.

Discussion
The present study evaluated the physical properties of AH Plus, a two-component-paste root canal cement that is based on the polymerization reaction of epoxy resin amines, and MTA Fillapex, a MTA-based sealer composed of resins, radiopaque bismuth, nanoparticulated silica, and MTA. The physical properties of a material give important information about its applicability. Thus, setting times, flow, solubility, dimensional stability, and radiopacity were measured for each sealer, in terms of ISO 6876/2001 25 . Moreover, a photoelastic analysis was conducted to evaluate how the sealers could generate stress on root dentin. The hypothesis of the study was rejected because the materials showed the same behaviors for all of the measured properties.
The setting time is the time necessary for the sealer to achieve its definitive properties 27 . It depends on the constituent components, particle size, ambient temperature, and relative humidity 28 . According to ISO 6876:2001 25 , the setting time should not vary by more than 10% of the manufacturer's information. The minimum setting times provided by the manufacturers of AH Plus and MTA Fillapex are 8 hours and 2 hours, respectively. There is a lack of agreement in the literature about the initial setting time for AH Plus. Different studies show ranges between 8 hours and 20 minutes to 24 hours 29,30 . This variability in setting time might be influenced by the regions where the paste was collect from the tube; for example, higher values for the setting time were found when the sealer was collected from the beginning of the tubes 31 . In the present study, the initial setting of AH Plus was 10 hours and 38 minutes; the final setting was 11 hours and 5 minutes. The mean values for MTA Fillapex was 3 hours and 51 minutes for the initial setting and 7 hours and 56 minutes for the final setting. Therefore, the setting times of both sealers in this study were greater than the minimum values reported by the manufacturers, but they were in accordance with ISO specifications. AH Plus's longer setting time was most likely due to the polymerization process, which is based on the polymerization reaction of epoxy resin amines that occurs gradually 32 . On the other hand, the lower setting time presented by MTA Fillapex can be attributed to the effects of butyl ethylene glycol disalicylate. This chemical compound can be used to obtain resins or polymers with different physical characteristics as rheological characteristics and handling properties. However, further studies are necessary to determine the actual effects of this compound on MTA Fillapex 22 .
A sealer's ability to flow and penetrate into irregularities and accessory canals and between gutta-percha cones is an important property 29 . However, an endodontic sealer must have a moderate flow rate because an excessive flow increases the chance that the material will extrude toward the periapical region, which may compromise periapical healing 33 . In the present study, both of the tested sealers exhibited the minimum diameter requested by ANSI/ADA 57:2000 26 , which is consistent with other studies [34][35][36] . MTA Fillapex had a significantly higher flow value than that of AH Plus. In general, factors such as the composition, particle size, shear rate, temperature, and time from mixing are mainly associated with the flowability characteristics of endodontic sealers. Furthermore, the resin to MTA ratio can influence flowability and, perhaps, justify the high flow values of MTA Fillapex 22 .
Solubility is the ability of a substance to dissolve in another, and it is expressed as the concentration of the saturated solution of the former in the latter 37 . According to ISO 6876:2001 25 , no more than 3 wt % of a cured root canal sealer should dissolve in water. Materials with a higher solubility may release irritants and increase the risk of leakage and bacterial colonization 37 . The solubility of the tested sealers were in agreement with ISO 6876, though MTA Fillapex exhibited a significantly higher solubility than AH Plus. In our study, the results for AH Plus were consistent with those of previous studies 32 ; however, in the present study, the range was higher than those of the previous studies. MTA Fillapex presented a lower solubility value when compared with previous studies that reported values between 14.22 and 14.94% 38 . According to Viapiana et al. 38 , MTA Fillapex's high solubility is likely due to the additives that are incorporated into the composition of the sealer, which likely destabi-lize its matrix. In addition, the presence of MTA and bismuth trioxide can promote negative effects on solubility. It is important to consider that solubility tests are performed after the sealers are set in a controlled ambience. These characteristics do not occur in clinical conditions in which some degree of humidity inside the root canal may exist, and fresh mixed sealer is used during the obturation procedure. Therefore, solubility in a clinical situation is probably higher 37,38 . Moreover, it is worth mentioning that the disintegration process should be considered as well when the solubility properties are analyzed. Disintegration is the act or effect of falling apart, the separation of something from a whole. When solubility is tested, there is no particle in suspension. However, in disintegration tests, there is a release of particles that remain in suspension 37 . In our study, analyzing the deionized distilled water used for the dimensional stability test resulted in a greater residue release of MTA Fillapex than AH Plus. This result indicates that the solubility of AH Plus is lower than that of MTA Fillapex. The performance of AH Plus might be related to the characteristics of its resinous matrix, which is more resistant to the solubility 3 .
Dimensional stability demonstrates the shrinkage or expansion of the material following setting. A primary role of a root canal sealer is to act as a physical blockage from invading bacteria originating in the oral cavity. In this perspective, sealers and other components of the filling should ideally be volumetrically stable or increase slightly 3,32  Root canal sealers should ideally have some degree of radiopacity in order to detect its placement and to be easily distinguishable from dentin and gutta-percha on radiographs. ISO 6876:2001 25 stipulates that the radiopacity should be higher than the radiopacity of 3 mm of aluminum. AH Plus and MTA Fillapex were found to be in agreement with ISO 6876:2001 25 . A statistical analysis showed that AH Plus had a significantly higher radiopacity than MTA Fillapex. These results were most likely due to the presence of different radiopacifying agents in each material. AH Plus has zirconium oxide, iron oxide, and calcium tungstate in its composition, which may account for the superior radiopacity 38 . The bismuth trioxide in the MTA Fillapex is responsible for its lower radiopacity 38 .
Only a few studies have used photoelastic stress analyses for evaluating root canal sealers, most likely because this test is not covered by ISO recommendations. However, it is a reliable method for determining how root canal sealers can generate stress in the root dentin. Overall, the intensity of the developed stress is associated with three principal factors: cavity geometry (C-factor), material characteristics, and restorative technique 39 . In the present study, AH Plus showed a lower polymerization stress when compared with MTA Fillapex. Apparently, the linear contraction presented by MTA Fillapex generated higher stress than the expansion of AH Plus. In general, if a filling material expands, there is a risk that the root may fracture. The setting expansion of root canal sealers induces radial pressure on the pulpal aspect of the dentin. The risk of fracture is due to the associated tangential strain, the magnitude of which is governed by the elastic modulus of dentin and the filling, the percentage expansion of the filling, and the tensile strength of dentin. Therefore, the consequences of a limited expansion by the sealers are highly dependent on the materials 3 . In addition, the placement of gutta-percha, which is a low-modulus material, reduces the volume of the root canal affected by sealer expansion and can be expected to absorb some of the generated stress 3 . On the other hand, contraction appears to be a less desirable property in these materials. Contraction due to polymerization is not problematic when it occurs on an unbonded surface; however, stress is generated at a cavity interface as a result of the forces generated by the contraction of the composite bonded to the dental structure 39 . Polymerization shrinkage leads to many clinical problems, such as marginal staining, recurrent caries, and restoration failure at the composite or tooth interface 39 . With a requirement of 1% maximum shrinkage, it only takes a material thickness of 100 mm to produce a void of 1 mm, which is enough space for bacterial penetration and growth. Thus, an insufficient root filling and shrinking material may be more likely to threaten success than a slightly expanding root canal sealer.
In conclusion, MTA Fillapex and AH Plus complied with ISO 6876:2001 25 , in terms of setting times, flow, solubility, dimensional stability, and radiopacity. MTA Fillapex showed a higher flow profile, solubility, and polymerization stress when compared with AH Plus. Further studies should aim to obtain more insight into the physical, mechanical, and chemical properties of the root canal sealer and how best to use them in specific clinical procedures.